This invention relates to compositions based on blends of polyamide and vinyl aromatic resins, particularly such compositions having improved compatibility and resistance to fibrillation.
Polyamide resins have long been known for their excellent toughness, flexibility, abrasion resistance and relatively high impact strength. Molded or extruded polyamides in general have found applications in appliances, consumer products, electronics or machine components, automotive parts, gears and like uses. In addition, spun fibers or filaments are used in textile and automotive tires as reinforcing materials.
Although the properties of the most popular polyamides today such as polyhexamethylene adipamide (nylon 66), polycaprolactum (nylon 6), poly (11-aminoundecanoic acid) (nylon - 11), or various nylon copolymers render these materials desirable in most applications, these and other polyamide polymers may suffer certain drawbacks in terms of lack of relative stiffness, relatively low Young's Modulus, and hygroscopicity.
It has been proposed in the prior art to modify the properties of polyamide resins by forming blends with other resinous materials, which have certain desirable properties not inherent in the polyamides, without sacrificing to any great extent the inherently good mechanical properties of the polyamide. For example, according to U.S. Pat. No. 3,093,255, hydrocarbon polymers such as polyethylene have been blended with polyamide resins to yield compositions that have improved permeability properties and are processible into films, filaments and bottles. Improvements in moisture resistance, impact strength, flexibility or molding characteristics have been obtained by forming blends of polyamides with graft or random copolymers of a monoolefin and a carboxylic acid or acid ester, as disclosed in U.S. Pat. Nos. 3,236,914 or 3,472,916, or by forming blends of a polyamide, a polyolefin and an olefin/carboxylic acid copolymer as disclosed in U.S. Pat. No. 3,373,223.
Polystyrene or styrene copolymers have long been known for their extremely good processing characteristics, moisture resistance and stiffness, although suffering from relatively poor mechanical properties when compared with the polyamides. Attempts to form blends consisting of polystyrene and polyamide resins to impart the desirable properties of each in a composition have not been entirely successful because these materials are incompatible. For example, blends consisting essentially of a polyamide and polystyrene have been prepared and extruded into rods only to find that the rods tend to fibrillate or split when subjected to mechanical strain. This fibrillation is caused by a splitting at the phase interfaces of the non-compatible resins in the blend. Although compatibility problems have been minimized in melt spun fibers based on polystyrene/polyamide blends by devising special spinning and quenching techniques, as taught for example in U.S. Pat. No. 3,330,899, or by incorporating polyamide/polystyrene graft copolymers into the blend, as taught by U.S. Pat. No. 3,334,153, the fibrillation problem is not so readily resolved where blends are processed into shaped goods such as by extrusion and by compression or injection molding operations.
Accordingly, it is an object of the present invention to prepare compositions comprising blends of polyamide and vinyl aromatic resins having improved resistance to fibrillation.
Another object is to prepare polyamide resin compositions having improved stiffness and water permeation properties.
Another object is to prepare vinyl aromatic resin compositions having improved flexibility and modulus characteristics.